In order to cool electric motors and power electronic inverters, two-phase cooling with the coolant changing from the liquid phase to the vapor phase is far more effective than using single-phase, such as a liquid to liquid heat transfer. The significant latent heat associated with two phase heat transfer makes two-phase cooling so attractive. Two-phase cooling answers the needs for increasing power density and to cool higher heat fluxes in inverters and traction drive motors.
Currently there are two cooling liquids available in an internal combustion engine vehicle. One is the 105° C. ethylene glycol/water supply obtained from the radiator. The other one is the 85° C. transmission oil. Strictly speaking, there is also refrigerant at high pressure available for passenger compartment air conditioning. Most people with the experience of their expensive household air-conditioning bills would have an impression that cooling the motors and inverters with a technology that is related to an air conditioning system would be impractical and expensive. Floating loop refrigerant cooling teaches that this is not necessarily true when the floating-loop system is used to cool the motors and inverters in conjunction with the air-conditioning system of a vehicle.
The cooling of various loads in a vehicle is currently conducted in a piece-meal fashion. An independent cooling system is used for the passengers. The major drive system of a hybrid or an electric vehicle contains a power electronic inverter. The direct current (DC) link capacitor is the most expensive and also the most bulky item in an inverter. To date, innovations in the thermal management of electric capacitors utilized in power electronics have been relatively limited. They have traditionally been restricted to single-phase cooling techniques, such as fans or heat sinks cooled by water or air. Although the two-phase-cooling heat pipes are available for capacitors, their cooling effects are limited as compared with the technology introduced in this invention.
In order to reduce the size of the DC link capacitor, the current invention teaches that by choosing the capacitor material to be compatible with the refrigerant, the capacitor can be in direct contact with the refrigerant. Unique two-phase cooling technique includes various surface treatments, spray, impingement, and capacitor arrangements. The permissible ripple current can be raised several times that of capacitors without direct refrigerant cooling. The use of this technology results in a smaller and less costly inverter.